Allow customization of default scheduler name on JVM

[IgnatBeresnev]

Fixes #3231

[IgnatBeresnev]

Didn't find any documentation for existing configurable properties (such as kotlinx.coroutines.scheduler.max.pool.size).

On one hand, I think it's useful and can be added to Dispatchers and threads section under Dispatchers.Default heading, for instance

On the other, maybe it's not desirable to advertise these properties? It only works on JVM, and the majority of users probably don't need it.

Wdyt? I can add documentation for all current properties if requested.

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Also, didn't find an easy way to write a unit/integration test for setting the name. Tell me if there's a way and I'm missing something :) I know there's a long way with configuring gradle tasks, but it doesn't seem rational for this type of change.

[dkhalanskyjb]

We typically do not advertise these properties, yes. The overwhelming majority of users never need to even think about them, and we're trying to keep the documentation relevant.

Also, yes, we don't write integration tests for these, they are very self-evident and are part of code that barely ever changes. It's enough to check manually if the property works.

[dkhalanskyjb]

Thanks!